In thermal power plants condensers have been traditionally used for cooling the exhaust gas (steam) of a steam-turbine to condense it and recycle the condensed water. In a condenser of such use thousands of or sometimes tens of thousands of long tubes of copper alloy, for example 5-40 m in length, having a small inner diameter on the order of 10-40 mm are incorporated as condenser tubes. Those tubes, which pass the cooling water such as sea water therethrough, are adapted to cool the exhaust steam passing thereoutside.
Those condenser tubes, which constantly pass the cooling liquid such as sea water containing many corrosive substances at a fairly high flow speed, for example 1-2.5 m/sec., are susceptible to various types of corrosion or erosion. It is therefore necessary to cover or coat the whole inner (internal) surface of those tubes with a protective synthetic resin coating or paint for the purpose of corrosion-and-rust resistance. The coating must avoid degradation of the heat transfer qualities, which is of course the essential function of the condenser tubes. Thus, a thin on the order of 10-30.mu. and uniform filmy coating which will not deteriorate the heat transferring or exchanging capability is needed for the protective coating of a heat conducting tube such as a heat exchanger tube in a heat changer like a condenser.
Such tubes having a thin filmy coating in the interior thereof is sometimes necessitated to repaint or recoat, before the life of the whole plant for example 20 or 30 years comes to an end, because the thin coated film (a) may be worn away after the tubes being installed in the plant to expose the ground metal by a mere aging, (b) may be eroded by shells or sand grains contained in the sea water, or (c) may be worn pre-maturely by the so-called sponge ball cleaning method used to remove foreign matters stuck to the tube such as seaweeds. The interior coating of resin paint film of the tubes are sometimes shorter in life than the plant itself, and it must be periodically or occasionally repainted (recoated) particularly in an installed state in the plant.
Conventionally practiced methods, mostly used for painting the interior of relatively short tubes ranging the whole length thereof, that is, flow coating method of flowing paint through a tube or brush coating method of brushing paint directly on the inner surface of the tube, have been defective for being applied to long tubes of small diameter in being difficult in getting a uniform-thick coated film. Particularly the former method is not good when it is applied to a tube already installed in a condenser, because the tube can not be inclined for flowing out the superfluous paint. Both are as a matter of fact hardly recognized of their practicality at the present stage.
As another method of relatively high practicality a spray coating method, wherein nebulized or atomized paint by a spray gun is coated on the inner surface of a tube, can be named. Sometimes a long necked spray gun with a length of 500 mm or so is inserted in the tube, but it is not free from a problem that the length of the tube to be effectively coatable by this spray gun is naturally restricted to some extent. Still another method, as a variation of the above, for avoiding the problem, is relatively widely practiced, wherein a moving nozzle which is shiftable from one end of a tube to the other end while spraying the paint is employed. In any way problems are still left unsolved as to what length of the tube interior can be well coated by the spray coating method. Even the latter, when it is applied to coating of the interior of a condenser tube of small diameter and large length, particularly in a heat exchanger tube of a condenser, leaves something to be desired. For example, as the coated film is apt to be largely influenced by temperature, humidity and other environmental conditions, it is very difficult to keep the film thickness at a desired uniform value in coating an already installed long tubes of small diameter where the necessary environmental conditions are almost out of control; uneven thickness or defective coating of the film may as a result take place there. When the tube to be coated is a long heat exchanger tube, varying the thickness of the coated film is likely to cause variation of the heat conducting or transferring capability. Thus, strict control of the environmental conditions for keeping the coating thickness even is of great importance.
Generally the thickness (t) of a coated film is regulated by an undermentioned formula, ##EQU1## wherein, q: discharge amount of the paint
.alpha.: solid ratio of the paint PA1 Di: inner diameter of the tube to be coated PA1 v: shifting rate of the spray nozzle PA1 .rho.: density of the coated film.
That is to say, the thickness (t) of the film is given as a function of the discharge amount (q) of the paint, solid ratio (.alpha.), i.e. of the solidified portion (component to be remained), of the paint, and the shifting rate (v) of the spray nozzle. Out of those (q) and (v) can be easily made constant independently of the environmental conditions of coating, but (.alpha.), the ratio of the solidified portion of the paint, is determined by the mixing ratio of the synthetic resin, pigment, and solvent. Coating is however practiced generally, irrespective of the environmental temperature, at a most suitable paint viscosity for spraying. The viscosity of a film forming substance such as synthetic resin depends, on the other hand, upon the temperature, so it is necessary to vary the mixing ratio of the solvent in the paint according to the environmental temperature under which the coating is carried out so as to keep a constant viscosity of the paint to be coated. In other words, the value .alpha. in the general formula mentioned above is varied to consequently change the thickness (t) of the film.
Observing this problem from the view point of defects of the coated film, operations at the plants in cold districts or in a winter season draw particular attention. The solvent ratio must inevitably be raised in such cases because of the remarkable low level of the ambient temperature for getting the predetermined viscosity of the paint. This consequently results in dripping or gathering of the paint toward the lower side of the tube due to elongation of the film forming time duration, remelting of the once solidified film in response to increasing of the solvent evaporating amount, and undesirable environmental pollution due to the evaporation of the solvent in large quantity.
Hot spray coating was proposed, on the other hand, to eliminate those disadvantages. Traditional technology of heating the paint or the air employed at the source thereof, be it by the paint heating method or the hot air spray method, is very impracticable from the view point of applying the same to the inner surface of long tubes of small diameter. Because, in the condenser tubes of large length already installed in a condenser or the like the distance from the paint reservoir to the end of the spray nozzle is not less than 20 m at the least, and consequently maintaining the temperature of the paint at a predetermined level is difficult. In case of the hot air spraying method supply of the required hot air of large quantity, such as 200-500 l/min., passing such a long distance needs a huge equipment for elevating and maintaining the temperature to and at a necessary level. This is the Achilles heel of the hot spray coating method in the practical application thereof.
An ideal method for coating a thin film of uniform thickness to the inner surface of a long tube of small diameter has not been established. Particularly in case of an already installed condenser tube in the condenser protective coating is confronted at the present stage with many technical difficulties. Actually the tubes which have been worn away of the coating due to the causes mentioned above in the running condition, have to be replaced by new ones completely coated, which causes a huge amount of working and material cost, bringing about a great loss.